The present invention relates to a revolver type developing device for a copier, laser printer, facsimile transceiver or similar image forming apparatus.
Conventional image forming apparatuses include a full color copier which exposes a photoconductive element or similar image carrier to color-separated light images to electrostatically form corresponding latent images, develops each of the latent images by a toner of complementary color, and transfers the resulting toner images to a single recording medium one above the other. A multicolor image forming apparatus is also conventional which sequentially exposes an image carrier to images to be reproduced in different colors, develops each of the resulting images by a developer of particular color to produce a corresponding toner image, and transfers such toner images to a single recording medium. This kind of image forming apparatuses need a plurality of developing units. However, a plurality of developing units constructed independently of each other and arranged around the image carrier would increase the overall size of the apparatus. A revolver type or rotary type developing device which is a recent achievement can eliminate this problem. The revolver type developing device, or simply revolver as referred to hereinafter, has a rotatable cylindrical casing located to face an image carrier, and a plurality of developing units disposed in the casing and supported in predetermined positions. The developing units are sequentially brought to a developing position to develop latent images by respective toners.
A prerequisite with the revolver type developing device is that a developer transport member accommodated in each developing unit and implemented as a roller or a sleeve be accurately positioned and fixed in a predetermined position in a developing region where it faces an image carrier. To meet this requirement, a groove may be formed in the outer periphery of the casing or rotary body and receive a pin, roller or similar stop provided at the outside of the casing, as disclosed in Japanese Patent Laid-Open Publication Nos. 3162/1986 and 78175/1988 by way of example. However, the problem with this kind of scheme is that the force restricting the position of the casing is not sufficient in the direction in which a spring for absorbing the inertial rotation of the casing expands or in the direction in which the casing rotates for the replacement of a color. Hence, when an external force or vibration acts on the casing, the casing is apt to rotate or vibrate away from the predetermined position. Further, a solenoid or similar extra mechanism, part and operation control are necessary for the stop to be released from the groove before the start of color replacement.
To position and fix the casing in the predetermined position, a pin movable perpendicularly to the end of the casing may be used and selectively inserted into a hole formed in the end of the casing, as taught in Japanese Patent Laid-Open Publication No. 65276/1986. However, this approach is not practicable unless the positional accuracy of the pin and hole is extremely high and without resorting to a solenoid or similar extra part for moving the pin. Moreover, to insert the pin into the hole which rotates together with the casing, extremely complicated control is needed over the rotation of the casing.
In the light of the above, a stepping motor or pulse motor may be used as an exclusive drive source for the casing or rotary body. Then, the casing will be positioned and fixed by the position control and holding ability of the motor itself. Alternatively, braking means may be associated with an exclusive motor for holding the casing at the predetermined position. A problem with these schemes is that a current has to be continuously fed while the casing is held in the predetermined position. Another problem is that a large size motor is needed to exert a sufficient holding force on the casing. As a result, the device generates heat, consumes disproportionate power, and increases the cost.